Hi there and welcome.
The 105.1 is really an easy load. and though they will improve with better amps, 50W should be enough to get a decent sound since they are not really low impedance.

So what can we theorise?
Punch is subjective, 105 bass is low colouration so bass will depend on the program and expectations.

Secondly bass clarity will depend on positioning and room, this can have a big effect, they need space around them to image well.

Thirdly there is a fault, push the B300 in evenly round the dust cap to its limit and let go, it should return to centre v-e-r-y slowly. If not check the sealing of the B300 baffle and the box internals around the cardboard tube? If the drivers still seem ok it is possible crossover capacitors may have drifted after this length of time? As an experiment short out the series bass capacitor of 360uF in the lower cabinet? Download a test frequency sweep or pink noise in octaves run and see if anything is obviously wrong?

Welcome here,
My 105.1 didn't need too much power before the couch I was sitting on started to vibrate in sync. The room is the key. In a small room you eventually have speaker/listener positioning combinations that can act like a "bass sink".

Also, the R105 are not disco style boom boxes. They can sound like that if you boost the bass or press the loudness switch but they are designed to give you flat, clean, response down to the 35Hz region.

These are without doubt the finest vintage KEF system pre-1990 I've had the pleasure of owning. Congratulations on your purchase!

From my experience I can suggest that you may need to give attention to the crossovers. A capacitor replacement could make all the difference you're hoping for.
The bass section caps would be the ideal place to test first. (They are located next to the woofer on the bass section panel .. easy to access).
My pair, so recapped, gives the most pleasing bass I could want. Having said that ... they do NOT work so well in any home theater set up without some LF help from a sub-woofer. These are Music Only speakers.

There are many great posts here in this forum on the 105's worth scouring for.
Please let us know how you make out with them. Operating properly they WILL grow on you _________________Music is food for the soul.

What replacement caps did you use, if I may ask?_________________Sometimes I lie awake at night, and I ask, "Where have I gone wrong?" Then a voice says to me, "This is going to take more than one night."
-- Charles M. Schulz (as Charlie Brown)

You could use polypropylene or polyester but would need to add some set on test resistors in the mid and treble to account for the lower internal loss factors. This is what I did many years ago when able to accurately measure the originals before any drift had set in and replace with better than 0.5% polyprops.

here the crossovers are under the 3 power amps in a separate box and triwired (actually quad wired with the indicator) to the drive units. Without knowing the original values there will be some inaccuracy since Kef used computer matching of tolerance bands to obtain sets very close to the design centre result. But the Falcon 2% should be the best fit and last for the next decade or two.